Water tends to collect at the bottom of aircraft fuel tanks, particularly on aircraft which are subjected to significant changes in pressure or humidity. Aircraft may therefore be equipped with a water drain valve through which excess water may be drained.
During flight or in cold weather conditions, an aircraft can experience very low temperatures. This can cause the water close to the cold wall of the fuel tank to freeze. Known water drain valves tend to fill, at least partially, with water and some of this water that is close to the cold wall can freeze, thereby jamming the valve. In this situation the fuel tank cannot be drained and draining must be delayed until the water in the valve has thawed. Any delay or restriction in aircraft maintenance is, of course, undesirable.